Cationic silicon (ii) compounds and method for the production thereof

ABSTRACT

The invention relates to a method for producing cationic silicon(II) compounds of general formula I 
       (R a Si) + HA −   (I)
         by reacting the silicon(II) compounds of general formula II       

       (R b- H)(R a Si) +   (II)
         with a hydride acceptor compound A,   wherein R a , (R b- H) and A have the definitions described in claim  1;  
 
to the cationic silicon(II) compounds of general formula I and use thereof as catalysts.

The invention relates to cationic Si(II) compounds and a method for the production thereof and also use thereof as catalysts.

Cationic silicon (II) compounds are highly reactive compounds, which are of industrial interest owing to their electronic structure, especially for catalytic purposes. It could be shown, for example in Angew. Chem. 2011, 123, 6975, that the cationic Si(II) compound (C₅Me₅)Si⁺B(C₆F₅)₄— catalyses the conversion of ethers with high selectivity.

In particular, the lack of synthetic accessibility has long stood in the way of the use of this compound class. The compound (C₅Me₅)Si⁺B(C₆F₅)₄— can be prepared from silicocene, (C₅Me₅)₂Si, —as described in Chem. Eur. Joc. 2014, 20, 9192—for example exclusively by reacting silicocene with the specific protic acid(C₅H₂Me₅)⁺ B(C₆F₅)₄—, which is accessible only in a very complex, in part safety-critical 7-stage low temperature synthesis according to Organometallics 2000, 19, 1442 in connection with Science, 2004, 305, 849. Protic acids that are easier to access always result, as stated in J. Organomet. Chem. 1993, 446, 139, in oxidative addition with formation of an adduct with tetravalent silicon.

Thus, there is a need for a simpler method by which cationic silicon(II) compounds can be made accessible.

The invention relates to a method for producing cationic silicon(II) compounds of general formula I

(R^(a)Si)⁺HA⁻  (I)

by reacting the silicon (II) compounds of general, formula II

(R^(b-)H)(R^(a)Si)⁺  (II)

with a compound A, where

-   R^(a) is a carbon-containing radical, which may additionally     comprise heteroatoms such as nitrogen, oxygen or sulfur, bound to     the cationic silicon atom via a covalent bond, ionic bond or via one     or more π bonds with the cationic silicon atom, -   (R^(b-)H) is a π-bonded singly negatively charged radical of general     formula III,

-   R^(x) are monovalent or polyvalent organic radicals, which may also     be bonded to one another to form a fused ring, with the proviso that     at least one of the radicals R^(x) is an organic radical bonded via     carbon bearing at least one hydrogen atom, preferably in the alpha     position to the ring, and -   A is a compound which, as an electrophile, is capable of accepting     electron pairs.

It has been found that, surprisingly, cationic silicon(II) compounds can be prepared by transferring a negatively charged hydrogen atom, a hydride ion, to a hydride acceptor. Hydride acceptors are very readily accessible synthetically. In this manner, therefore, the accessibility of the cationic silicon(II) compounds is substantially simplified. A further advantage is that the reaction takes place at ambient temperature in high yield without forming by-products.

An example of the reaction that takes place in the method is elucidated by using a preferred cyclopentadienyl radical (R^(b)—H) of general formula III, namely (Cp-CHR¹R²)⁻.

In the method, a hydride ion is transferred to the hydride acceptor A forming HA⁻ and the compound Cp=CR¹R² (corresponds to R^(b)) according to reaction equation 1,

(Cp-CHR¹R²)⁻(R^(a)Si)⁺+A=>Cp=CR¹R²+(R^(a)Si)⁺+HA⁻  (1),

where Cp is a cyclopentadienyl radical.

The compound HA⁻ that is also formed in the reaction forms the counterion to the cationic silicon(II) compound (R^(a)Si)⁺.

The radical R^(a) is preferably π-bonded and comprises at least two conjugated carbon-carbon double bonds.

-   R^(a) is especially preferably a π-bonded cyclopentadienyl radical     substituted by the radicals R^(y) (R^(y) ₅Cp) where -   R^(y) are monovalent or polyvalent organic radicals, which may be     bonded also to one another to form fused rings.

In this case, the compound of general formula I has the structure of general formula Ia:

The radicals R^(a) and R^(b) may also be bonded to each other, preferably via one or more carbon radicals.

The radicals R^(y) are preferably each independently hydrogen, linear or branched, acyclic or cyclic, saturated or mono- or polyunsaturated C1-C20 alkyl or C6-C20 aryl, particularly preferably C1-C3 alkyl, especially preferably methyl radicals. Example of radicals R^(y) are alkyl radicals such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, neopentyl or tert-pentyl radical; hexyl radicals such as the n-hexyl radical; heptyl radicals such as the n-heptyl radical; octyl radicals such as the n-octyl radical, and isooctyl radicals such as the 2,4,4-trimethylpentyl radical; nonyl radicals such as the n-nonyl radical; decyl radicals such as the n-decyl radical; dodecyl radicals such as the n-dodecyl radical; hexadecyl radicals such as the n-hexadecyl radical; octadecyl radicals such as the n-octadecyl radical; cycloalkyl radicals such as the cyclopentyl, cyclohexyl or cycloheptyl radical and methylcyclohexyl radical; aryl radicals such as the phenyl, naphthyl, anthryl and phenanthryl radical; alkaryl radicals such as the o-, m- and p-tolyl, xylyl, mesitylenyl and o-, m- and p-ethylphenyl radical; and aralkyl radicals such as the benzyl radical, the α- and β-phenylethyl radical.

The radical (R^(b−)R) is a cyclopentadienyl anion, which consists of a singly negatively charged aromatic five-ring system C₅R^(x) ₅—, substituted by the radicals R^(x).

The radicals R^(x) in the compound of general formula III are preferably each independently hydrogen, linear or branched, acyclic or cyclic, saturated or mono- or polyunsaturated C1-C20 alkyl or C6-C20 aryl, particularly preferably C1-C3 alkyl, especially preferably methyl radicals.

At least one of the radicals R^(x) is preferably a group —CHR¹R², wherein the radicals R¹ and R² are each independently hydrogen, linear or branched, acyclic or cyclic, saturated or mono- or polyunsaturated C1-C20 alkyl or C6-C20 aryl, particularly preferably C1-C3 alkyl, especially preferably methyl radicals. Examples of radicals R^(x) are alkyl radicals such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, neopentyl or tert-pentyl radical; hexyl radicals such as the n-hexyl radical; heptyl radicals such as the n-heptyl radical; octyl radicals such as the n-octyl radical, and isooctyl radicals such as the 2,4,4-trimethylpentyl radical; nonyl radicals such as the n-nonyl radical; decyl radicals such as the n-decyl radical; dodecyl radicals such as the n-dodecyl radical; hexadecyl radicals such as the n-hexadecyl radical; octadecyl radicals such as the n-octadecyl radical; cycloalkyl radicals such as the cyclopentyl, cyclohexyl or cycloheptyl radical and methylcyclohexyl radical; aryl radicals such as the phenyl, naphthyl, anthryl and phenanthryl radical; alkaryl radicals such as the o-, m- and p-tolyl, xylyl, mesitylenyl and o-, m- and p-ethylphenyl radical; and aralkyl radicals such as the benzyl radical, the α- and β-phenylethyl radical.

Compounds A are hydride acceptors. Preferred compounds A are MX₃ where M=B, Al and Ga and MX₅ where M=P, As, Sb, Ir and Pt. The radicals X each independently have the definition of an optionally substituted saturated, unsaturated, acyclic or cylic optionally substituted carbon or carbonoxy radical or have the definition of halogen.

Particularly preferred radicals X in the compounds MX₃ are C1-C20 alkyl, C1-C20 alkoxy, C6-C20 aryl or C6-C20 aryloxy radicals, which are substituted by halogen, preferably fluorine, or fluorine, chlorine and bromine.

Particularly preferred radicals X in the compounds MX₅ are fluorine, chlorine and bromine.

Preferred examples of compounds A are:

BF₃, BCl₃, AlF₃, AlCl₃, PCl₃, AsF₅, SbF₅, SbCl₅, PCl₅, PF₅, B(C₆F₅)₃, B(CF₃)₃, B(C₆Cl₅)₃, B(C₆H₅)₃, B(2,6-difluoro-C₆H₃)₃, B(2,4,6-trifluoro-C₆H₂)₃, B(2-C₆F₅—C₆F₄)₃, B(2,3,5,6-tetrafluoro-C₆H)₃, B(4,5,6,7-tetrafluoro-2-naphthyl)₃, Al(C₆F₅)₃, Ga(C₆F₅)₃, B[C₆H₂(CF₃)₃]₃, Al(OR^(PF))₃, FAl(OR^(PF))₂ and Sb(OTeF₅)₅.

The compound HA⁻ that is formed in the reaction according to reaction equation 1 forms the counterion to the cationic silicon (II) compound (R^(a)Si)⁺.

However, it is also possible to exchange the anion HA⁻ by other anions, for example by a resalting reaction which is downstream of the reaction according to the invention for example.

Also formed in the reaction is the cleavage product R^(b), which has the definition Cp=CR¹R² according to exemplary reaction equation 1. Cleavage products R^(b) may be left in the reaction mixture or can however be removed if this is advantageous, for example if R^(b) interferes in catalytic processes.

The removal can be effected in a manner known to those skilled in the art, for example by distillation or by fractional crystallization. The compound R^(b) is preferably distilled off in the distillative process, preferably under reduced pressure. If the removal is effected by crystallization, the compound of general formula I thus is preferably crystallized out by adding a precipitating agent and can be filtered off for example, the compound of general formula R^(b) remaining in solution.

The molar ratio of the compound of general formula II and the hydride acceptor A is preferably at least 1:10 and at most 10:1, particularly preferably at least 1:5 and at most 5:1, especially preferably at least 1:3 and at most 3:1. The two components can be mixed in any sequence here, wherein the mixing is carried out in a manner known to those skilled in the art.

The reaction according to the invention can be carried out in the presence of one or more further components, for example, in the presence of one solvent or a mixture of two or more solvents.

Either the compound of general formula II or the compound A or both components can be dissolved in one solvent or in a solvent mixture. The proportion of solvent or solvent mixture, based on the sum of the compounds of general formula II and A, is preferably at least 0.1% by weight and at most 1000-fold the amount by weight, particularly preferably at least 10% by weight and at most 100-fold the amount by weight, especially preferably at least 30% by weight and at most 10-fold the amount by weight.

The solvents used can be, for example, hydrocarbons such as pentane, hexane, heptane, cyclohexane or toluene, chlorohydrocarbons such as dichloromethane, chloroform, chlorobenzene or 1,2-dichloroethane, ethers such as diethyl ether, methyl tert-butyl ether, anisole, tetrahydrofuran or dioxane, or nitriles such as acetonitrile or propionitrile.

The reaction according to the invention to give the compound of general formula I may also be carried out in the presence of components which react in the presence of the compound of general formula I. The compound of general formula I effective as catalyst for the reaction of the further components is in this case generated in the presence of the reactants.

The reaction can be carried out at atmospheric pressure or under reduced pressure or under elevated pressure.

The pressure is preferably at least 0.01 bar and at most 100 bar, particularly preferably at least 0.1 bar and at most 10 bar, the reaction being carried out especially preferably at atmospheric pressure.

The reaction according to the invention is preferably carried out at temperatures between at least −100° C. and at most+250° C., particularly preferably between at least −20° C. and at most+150° C., especially preferably between at least 0° C. and at most+100° C.

The invention further relates to the cationic Si(II) compounds of general formula I

(R^(a)Si)⁺HA⁻  (I).

Examples of compounds of general formula I are:

(C₅Me₅)Si⁺HB(C₅F₅)₃—, (C₅Me₅) Si⁺HBF₃—, (C₅Me₅) Si⁺HB(CF₃)₃—, (C₅Me₅) Si⁺HB(C₆Cl₅)₃—, (C₅Me₅)Si⁺HB(C₆H₅)₃—, (C₅Me₅) Si⁺HB (2,6-difluoro-C₆H₃)₃—, (C₅Me₅) Si⁺HB (2,4,6-trifluoro-C₆H₂)₃—, (C₅Me₅) Si⁺HB (2-C₆F₅—C₆F₄)₃—, (C₅Me₅) Si⁺HB(2,3,5,6-tetrafluoro-C₆H)₃—, (C₅Me₅) Si⁺HB(4,5,6,7-tetrafluoro-2-naphthyl)₃-, (C₅Me₅)Si⁺HB [C₆H₂ (CF₃)₃]₃—, (C₅Me₅) Si⁺HAl(C₆F₅)₃—, (C₅Me₅) Si⁺HGa(C₆F₅)₃—, (C₅Me₅) Si⁺Al(OR^(PF))₃—, (C₅Me₅)Si⁺HFAI (OR^(PF))₂—, (C₅Me₅) Si⁺Sb(OTeF₅)₅—, (C₅Me₅) Si⁺HBCl₃—, (C₅Me₅) Si⁺HAlF₃—, (C₅Me₅) Si⁺HAlCl₃—, (C₅Me₅)Si⁺_HPCl₃—, (C₅Me₅) Si⁺HAsF₅—, (C₅Me₅)Si⁺HSbF₅—, (C₅Me₅) Si⁺HSbCl₅—, (C₅Me₅)Si⁺HPCl₅—, (C₅Me₅)Si⁺HPF₅—, (C₅iPr₅) Si⁺HB(C₅F₅)₃— and (C_(a)iPr₅) Si⁺HBF₃—, (C₅H₂Me₃)Si⁺HB(C₆F₅)₃ ⁻, (C₅H₂Me₃) Si⁺HBF₃—.

The cationic silicon(II) compounds of general formula I can be used as catalysts, for example for conversion reactions of ethers, cationic polymerizations and hydrosilylations. As could be shown as in the example of hydrosilylation, processes that are catalysed by silicon(II) compounds of general formula I proceed particularly uniformly and without notable formation of by-products.

All aforementioned symbols of the formulae above are each defined independently of one another. In all formulae, the silicon atom is tetravalent.

Unless stated otherwise in each case, all amounts and percentages are based on weight and all temperatures are 20° C.

EXAMPLES

The reaction of silicocene, (C₅Me₅)₂Si with the hydride acceptor B(C₆F₅)₃ at 20° C. affords in quantitative yield the cationic silicon(II) compound (C₅Me₅)Si⁺HB(C₆F₅)₃— and tetramethylfulvene (C₅Me₄)=CH₂.

Example 1

All process steps are carried out under Ar. 20.1 mg (0.067 mmol) of decamethylsilicocene [(C₅Me₅)₂Si, formula II where R^(a)═C₅Me₅ and (R^(b)—H)═C₅Me₅—], and 30.0 mg (0.059 mmol) of tris(pentafluorophenyl)borane are each dissolved in 0.5 ml of CD₂C₁₂ and the solutions combined in an NMR tube. A dark yellow coloration is immediately formed which indicates the formation of tetramethylfulvene. The solution comprises the compound (Me₅C₅)Si⁺BH(C₅F₅)₃—.

The signals of this compound remain unchanged at 20° C. over a time period of 35 days, i.e. no conversion degradation takes place.

¹H-NMR (CD₂Cl₂): δ=1.83 and 1.88 (2s, 4 Me of tetramethylfulvene), 2.19 [s, (Me₅C₅)Si⁺], 3.57 [q, broad, J(HB)=17 Hz, H—B(C₅F₅)₃—], 5.40 (s, 2H, H₂C═ of tetramethylfulvene); ¹⁹F-NMR (CD₂Cl₂): δ=−133.6 (m, 2F); −164.1 (m, 1F); −167.2 (m, 2F);

²⁹Si—NMR (CD₂Cl₂): δ=−399.1.

Example 2

All process steps are carried out under Ar. 11.8 mg (0.046 mmol) of decamethylsilicocene [(Me₅C₅)₂Si] and 23.3 g (0.046 mmol) of tris(pentafluorophenyl)borane are weighed successively into an NMR tube and 1 ml of CDCl₃ is added. The two components dissolve and a dark yellow coloration is immediately formed which indicates the formation of tetramethylfulvene. The solution comprises the compound (Me₅C₅)Si⁺BH(C₆F)₃—.

¹H-NMR (CDCl₃): δ=1.76 and 1.82 (2s, 4 Me of tetramethylfulvene), 2.10 [s, (Me₅C₅)Si⁺], 3.48 [q, broad, H—B(C₆F₅)₃—], 5.40 (s, 2H, H₂C═);

²⁹Si—NMR (CDCl₃): δ=−399.9.

Example 3

All process steps are carried out under Ar. 100 mg (0.335 mmol) of decamethylsilicocene and 172 mg (0.335 mmol) of tris(pentafluorophenyl)borane are each dissolved in 0.5 ml of CD₂Cl₂ and the two solutions are combined. The solution immediately colors dark yellow, the reason being the formation of tetramethylfulvene. Heptane is added, whereupon a colorless crystalline precipitate of the compound (Me₅C₅)Si⁺BH(C₆F₅)₃— is formed. The suspension is concentrated a little under reduced pressure to remove CD₂Cl₂ and 1 ml of heptane is again added.

The solution is removed using a syringe and the crystalline residue is washed twice more with 1 ml of heptane each time. It is dried at 20° C. under high vacuum and 155 mg (64%) of the pure fulvene-free product (Me₅C₅)Si⁺BH(C₆F₅)₃— is obtained.

Example 4: Hydrosilylation Reaction

All process steps are carried out under Ar. 118.5 mg (1.00 mmol) of α-methylstyrene and 136.7 mg (1.00 mmol) of dimethylphenylsilane are dissolved in 0.5 ml of CD₂Cl₂ and, at 20° C., a solution of 2.8 mg (0.0041 mmol) of the compound (Me₅C₅)Si⁺BH(C₆F₅)₃— (prepared as in example 3) in 0.5 ml of CD₂Cl₂ is added. The hydrosilylation reaction that occurs spontaneously with formation of the product PhSiMe₂-CH₂—CH(CH₃)Ph is monitored by ¹H-NMR spectroscopy. After 20 minutes ca. 50% product has formed and the reaction is complete after 24 hours.

Example 5: Hydrosilylation Reaction

All process steps are carried out under Ar. To the NMR sample prepared according to example 1, comprising the compound (Me₅C₅)Si⁺BH(C₆F₅)₃— and tetramethylfulvene, is added 9.1 mg (0.067 mmol) of dimethylphenylsilane. The initially still dark yellow solution decolorizes within a few hours. The yellow-colored tetramethylfulvene reacts completely with dimethylphenylsilane to form the hydrosilylation product PhSiMe₂—CH₂—C₅Me₄ (NMR spectroscopic investigation). The compound (Me₅C₅)Si⁺BH(C₆F₅)₃— is verified in unchanged amount by ¹H-NMR spectroscopy (singlet at δ=2.2 ppm).

Example 6: Hydrosilylation Reaction

All process steps are carried out under Ar. To a solution in CD₂Cl₂ prepared according to example 1, comprising 0.033 mmol of the compound (Me₅C₅)Si⁺BH(C₆F₅)₃— and tetramethylfulvene respectively, is added a mixture of 17.4 mg (0.147 mmol) of α-methylstyrene and 14.3 mg (0.105 mmol) of dimethylphenylsilane. The products of the hydrosilylation reactions of dimethylphenylsilane with α-methylstyrene [PhSiMe₂—CH₂—CH(CH₃)Ph] and with tetramethylfulvene (PhSiMe₂—CH₂—C₅Me₄) are formed. 

1. A method for producing cationic silicon(II) compounds of general formula I, comprising: (R^(a)Si)⁺HA⁻  (I) reacting the silicon(II) compounds of general formula II (R^(b)—H)(R^(a)Si)⁺  (II) with a compound A, where R^(a) is a carbon-containing radical, which may additionally comprise heteroatoms, bound to the cationic silicon atom via a covalent bond, ionic bond or via one or more π bonds with the cationic silicon atom, (R^(b)—H) is a p-bonded singly negatively charged radical of general formula III,

R^(x) are monovalent or polyvalent organic radicals, which may also be bonded to one another to form a fused ring, with the proviso that at least one of the radicals R^(x) is an organic radical bonded via carbon bearing at least one hydrogen atom, and A is a compound which, as an electrophile, is capable of accepting electron pairs, wherein said compound has formula MX₃ where M is B, Al or Ga, and where X is a C6-C20 aryl radical substituted by halogen.
 2. A cationic silicon(II) compound of general formula I (R^(a)Si)⁺HA⁻  (I) where R^(a) is a carbon-containing radical, which may additionally comprise heteroatoms, bound to the cationic silicon atom via a covalent bond, ionic bond or via one or more p bonds with the cationic silicon atom and A is a compound which, as an electrophile, is capable of accepting electron pairs, having formula MX₃ where M is B, Al or Ga, and where X is a C6-C20 aryl radical substituted by halogen.
 3. The method of claim 1, wherein the radicals R^(x) are each independently hydrogen, linear or branched, acyclic or cyclic, saturated or mono- or polyunsaturated C1-C20 alkyl or C6-C20 aryl radicals.
 4. The method of claim 1, wherein R^(a) is a p-bonded cyclopentadienyl radical substituted by the radicals R^(y)(R^(y) ₅Cp) wherein R^(y) are monovalent or polyvalent organic radicals, which may be bonded to one another to also form fused rings.
 5. The method of claim 1, wherein said reacting the silicon(II) compounds of general formula II with the compound A is carried out in aprotic solvents selected from the group consisting of hydrocarbons, chlorohydrocarbons, ethers, nitriles, organosilanes and organosiloxanes.
 6. The cationic silicon(II) compounds of general formula I as claimed in claim 2, wherein said cationic silicon(II) compounds are catalysts. 